The invention relates to a method of preparing efficient photoresponsive devices and to devices such as photovoltaic devices and photodetectors prepared by this method.
Photoresponsive devices based on semiconductive polymers have been investigated for a number of years, examples of such devices can be found in WO 99/09603, GB 2,315,594, U.S. Pat. Nos. 5,523,555, and 5,670,791. In these disclosures polymer photoresponsive devices include an active polymer layer or layers situated between two electrodes. Where the two electrodes have different work functions a built-in, internal field is generated across the active polymer layer giving rise to a photovoltaic effect, such devices are termed photovoltaic cells. A device of this construction will also have rectifying properties so may also be termed a photodiode. Photoresponsive devices have application as solar cells which generate electricity from light and as photodetectors which measure or detect light.
Photovoltaic devices such as that described in U.S. Pat. No. 5,670,791 have a construction in which a layer comprising a blend of two semiconductive polymers having different electron affinities and/or ionization potentials is situated between two electrodes of different work function, FIG. 1 shows such a device having a transparent glass substrate (1), a transparent anode (2), a hole transport layer (3), a layer comprising a blend of two semiconductive polymers forming an interpenetrating network (4) and a metallic cathode (5). The electrodes of different work function set up an internal electric field across the device. Such devices operate on the principle that absorption of light by the polymers of the organic layer generates bound electron-hole pairs, termed excitons. Excitons generated on the polymer of lower electron affinity dissociate by transfer of an electron to the polymer of higher electron affinity, the polymer of lower electron affinity is sometimes referred to as the electron donor or simply donor. Excitons generated on the polymer of higher electron affinity dissociate by transfer of a hole to the polymer of lower electron affinity, the polymer of higher electron affinity is sometimes referred to as the electron acceptor or simply acceptor. The electrons and holes generated by dissociation of the excitons then move through the device, with electrons moving to the lower work function cathode and holes moving to the higher work function anode. This is shown in FIG. 2(a) which shows a photovoltaic device having an interpenetrating network of two semiconductive polymeric materials one of which has a higher electron affinity (41) than the other (42), FIG. 2(b) shows schematically an exciton (43) generated by an incident photon being split into an electron (44) and a hole (45), the electron moving through the polymer of higher electron affinity (41) and the hole moving through the polymer of lower electron affinity (42). In this way light incident on the device generates a current which may be used in an external circuit. Photoresponsive devices may also be constructed in which the electrodes are of the same work function and an external bias is applied across the device to fulfill the function of the above described internal bias.
Increasing the efficiency of photovoltaic devices is a primary concern in the solar cell industry. At present photovoltaic cells based on organic materials are characterised by low power conversion efficiencies and low external quantum efficiencies. The power conversion efficiency is the ratio of the power taken out of the device in the form of electricity to the power put into the device in the form of incident light. The external quantum efficiency is the ratio of electrons collected from a device to the number of photons incident on the device, this is also known as the quantum yield.
In WO 99/49525 a method for increasing the efficiency of a photovoltaic device is disclosed in which a component having an electrode and a layer of a semiconductive polymer which acts as an electron donor and component having an electrode and a layer of a semiconductive polymer which acts as an electron acceptor are laminated together in such a manner so as to form a device with a layer comprising a mixture of the two semiconductive polymers at the interface where the two components are laminated together. A device prepared in this way was found to have higher efficiency than a device prepared from a blend of two semiconductive polymers, such as the device disclosed in the aforementioned U.S. Pat. No. 5,670,791.
It has been observed in photovoltaic devices having a double layer structure in which the layers comprise the semiconductive polymer poly(3-dodecylthiophene) and the semiconductive molecule N,Nxe2x80x2-diphenylglyoxaline-3,4,9,10-perylene tetracarboxylic acid diacidamide or a mixture of the two, that annealing the double-layer structure device leads to improvements in device efficiency, see Feng et al, Journal of Applied Physics, 88 (12), 2000, 7120-7123.
Similarly a photovoltaic device comprising a blend of poly(phenylenevinylene) and a perylene derivative has been prepared, annealing the device was shown to increase the external quantum efficiency of the device, see Dittmer et al, Solar Energy Materials and Solar Cells, 61, (2000), 53-61.
The invention provides a method for greatly improving the power conversion efficiency of photovoltaic devices having a single layer of a blend of semiconductive polymers. The method employed has significant processing advantages in that it does not involve complex lamination steps and since it uses only polymeric materials the present invention has advantages over devices which include small molecule electron donors and/or acceptors in that the devices can be readily prepared by solution processing techniques such as spin coating. Further the annealing step is carried out after the layer of active polymeric material has been positioned between the two electrodes such that the electrodes to some degree prevent degradation of the polymeric material which may occur on annealing. The invention allows access to a wide range of photovoltaic devices of improved performance.
In a first embodiment, the invention provides a method of preparing an efficient photoresponsive device comprising the steps of forming a photoresponsive device by providing a first electrode on a substrate, providing a layer of organic material over said first electrode comprising a blend of at least two semiconductive polymers, the semiconductive polymers having different electron affinities and/or different ionization potentials, and providing a second electrode over the layer of organic material, wherein at least one of said electrodes is transparent or semi-transparent, and carrying out a further step in which the photoresponsive device is thermally annealed.
The semiconductive polymers used in devices according to the invention are preferably selected from the group of polymers and copolymers consisting of polyfluorenes, polybenzothiazoles, polytriarylamines, poly(phenylenevinylenes), polyphenylenes, polythiophenes, polypyrroles, polyacetylenes, polyisonaphthalenes and polyquinolines. The polymers and copolymers polyfluorene, polybenzothiazole, polytriarylamine, poly(phenylenevinylene), and polythiophene are preferred.
In a preferred embodiment, photoresponsive devices prepared by the method of the invention include at least one semiconductive polymer selected from the group consisting of polyfluorene and poly(phenylenevinylene) and at least one alkyl substituted polythiophene.
Particularly preferred polymers include polyphenylenevinylenes such as MEH-PPV (poly(2-methoxy, 5-(2xe2x80x2-ethyl)hexyloxy-p-phenylenevinylene)), MEH-CN-PPV (poly(2,5-bis(nitrilemethyl)-1-methoxy-4-(2xe2x80x2-ethyl-hexyloxy)benzene-co-2,5-dialdehyde-I-methoxy4-(2xe2x80x2-ethylhexyloxy)benzene)) and CN-PPV (cyano substituted PPV), polyalkylthiophenes, such as poly(3-hexylthiophene) and poly(3-dodecylthiophene), polyfluorenes, such as poly(2,7-(9,9-di-n-o-ctylfluorene), poly(2,7-(9,9-di-n-octylfluorene)-benzothiadiazole) and poly(2,7-(9,9-di-n-octylfluorene)-(4,7-di-2-thienyl-(benzothiazole)) and related polybenzothiazoles such as are disclosed in WO 01/49768. Examples of some of these polymers are shown in FIG. 3.
The method of the invention is applicable to photoresponsive devices comprising a blend of at least two semiconductive polymers, said semiconductive polymers having different electron affinities and/or different ionization potentials, the organic layer of the photoresponsive devices may comprise further organic and or inorganic species such as further semiconductive polymers or charge transport components. In a preferred embodiment the blend of semiconductive polymers of method of the invention comprises two semiconductive polymers, preferably in a ratio of 1:1.
The thermal annealing of the photoresponsive devices may be carried out at any suitable temperature, suitable temperatures may be readily determined by the person skilled in the art. It is preferred that the thermal annealing is carried out by heating said device at a temperature between 60xc2x0 C. and 220xc2x0 C., preferably between 100xc2x0 C. and 180xc2x0 C. The thermal annealing may be carried out for a period of from 10 minutes to several days, although preferably annealing is performed for between 30 minutes and 12 hours. The annealing time has been found to have little effect on the resultant improvement in device efficiency.
Although not essential to the performance of the invention it is preferred that said first electrode of the photoresponsive device comprises a material of high work function and said second electrode of the photoresponsive device comprises a material of low work function. Preferred high work function materials include indium-tin oxide, tin oxide, indium-doped zinc oxide, aluminum-doped zinc oxide, magnesium-indium oxide, cadmium-tin oxide, Au, Ag, Ni, Pd and Pt. Preferred low work function materials include Li, Na, K, Rb, Be, Mg, Ca, Sr, Ba, Yb, Sm, Al, and alloys comprising the aforementioned low work function materials. A layer of dielectric material may be disposed between said second electrode and said layer of organic material; preferably, this dielectric material is selected from oxides or halides of alkali metals or alkaline earth metals; fluorides of alkali metals or alkaline earth metals are preferred.
The substrate on which the first electrode of the photoresponsive device prepared according to the inventive method is deposited is preferably selected from the group of materials consisting of glass, acrylic resins, polycarbonate resins, polyester resins, and polyethylene terephthalate resins.
The method of the invention may also involve providing further layers in said photoresponsive device such as charge transporting and/or charge injecting layers, preferably a hole transporting layer is provided between said first electrode and the layer of organic material; the hole transport material is preferably poly(ethylenedioxythiophene):polystyrene sulfonic acid (PEDOT:PSS)
In a further embodiment, the invention provides a photoresponsive device obtained by the method of the invention. The photoresponsive device is preferably a photovoltaic device or a photodetector. Where said photoresponsive device is a photodetector, the device further comprises external means for measuring the current flowing across the device and for converting this into a signal.
The invention further provides a method of increasing power conversion efficiency and/or quantum yield in a photoresponsive device through the use of thermal annealing, whereby a photoresponsive device comprising a layer of organic material comprising a blend of at least two semiconductive polymers, the semiconductive polymers having different electron affinities and/or ionization potentials is thermally annealed.